As services have been diversified on networks, we have widely used those new services which make use of advantages of networks. A typical example of the movements is a broadcasting and communication converged service, that is, an integrated service of broadcasting, Internet, and telephone (voice communication), which is called a triple-play service. In order to realize the triple-play service, constitution of Fiber To The Home (FTTH) by the PON system is becoming the main stream in access networks. In the PON system, multiple subscribers commonly use the optical fibers installed from the central office to the optical splitters, and the equipment of the central office. This common use enables a significant reduction of the network installation cost and maintenance cost by sharing those costs among subscribers. The FTTH system using the PON technology is a media sharing type network system described above, and a bandwidth a subscriber can use is nearly the same as that of what the maximum throughput of the system is divided by the number of subscribers who share the system. Since all subscribers rarely access the system at a same time, a subscriber can substantially use a wider bandwidth due to the statistical multiplex effect. Such broadband performance of the FTTH system by the PON becomes important for practicing a comfortable triple play service. The current PON systems are as follows: GE-PON by ITU-T (ITU-T G.984.1 “Gigabit-capable Passive Optical Networks (G-PON): General characteristics”, ITU-T G.984.2 “Gigabit-capable Passive Optical Networks (G-PON): Physical Media Dependent (PMD) layer specification”, ITU-T G.984.3 “Gigabit-capable Passive Optical Networks (G-PON): Transmission convergence layer specification”); GE-PON (IEEE standard) (IEEE 802.3ah “CSMA/CD Access Method and Physical Layer Specifications Amendment: Media Access CONUrol Parameters, Physical Layers, and Management Parameters for Subscriber Access Networks.” For example, in the G-PON system, an apparatus of the central office (OLT; Optical Line Terminal) corresponds to a maximum of 64 sets of apparatuses of subscribers (ONU; Optical Network Unit) via 2.4 Gbps high-speed optical lines.
As a mechanism to make the above sharing of the OLT possible, the collision avoidance control is provided in the system. Each optical signal (upstream signal) output from each ONU, after passing through an optical splitter, is superposed one on another, the superposed signals being output toward the OLT. In order for the OLT to be able to receive these multiple signals separately, each signal from each ONU must reach the OLT at a different timing without each signal not being superposed one on another. For this purpose, the collision avoidance control is adopted in the system and an output timing of an optical signal is controlled. The standardization organizations (ITU-T, IEEE) have now started the study of the next generation PON to be a successor of these present PON systems. To make the PON system further broader, the TDMA method used in the present POM must be improved so as to have a further higher-speed and a higher bit rate.
The wavelength multiplex transmission technology is used in the triple play service in the PON system: wavelength range of 1550 to 1560 nm is assigned to the video transmission system; for the PON system, 1490 nm band data optical signal is assigned to optical signals of from the OLT to the ONT (downstream signals), while 1300 nm band data optical signal is assigned to optical signals of from the ONT to OLT.